There are many ways to embed Video into a HTML page - as Flash Video or instances of Platform-Specific players through <object>
and <embed>
tags (but not every one of those tags is a video! The same holds true for .swf
- it's just the file extension of Flash files, Video or not), the new HTML 5 <video>
tag... They are not impossible to find out but it's a lot of work to catch all possible player types, formats and embed codes, and will result in a lot of false positives / negatives.
Then, there are JavaScript libraries that initialize players after the containing page has loaded - those are almost impossible to detect.
It's still a very complex issue to get video into a web page reliably, and subsequently, it's even more complex to find it out. Depending on what you are trying to achieve, I would consider dropping it.